Timing is not everything, but if you are writing a book on the United States and Europe, it sure helps if the possible breakup of the trans-Atlantic relationship dominates headlines on both sides of the ocean. Robert Kagan's short book, which expands on a celebrated article published in Policy Review last summer, captures the mood of today's crisis between the United States and many of its old European allies. The immediate object of that crisis is the difference over Iraq and over what the best response is to the threat posed by terrorists, tyrants and the technologies of mass destruction more generally.

But the cause of the crisis lies much deeper. ''When it comes to setting national priorities, determining threats, defining challenges, and fashioning and implementing foreign and defense policies, the United States and Europe have parted ways,'' writes Mr. Kagan, concluding, in words already famous in another context, ''Americans are from Mars and Europeans are from Venus.''

As an American living in Brussels the past three years, Mr. Kagan is in an ideal position to dissect what is wrong in the United States-European relationship and why. He does so with a surgeon's skill, stripping away layer after layer to reveal what in the end is a remarkable conclusion: The West, which for so many decades defined the two sides of the Atlantic, has ceased to be the organizing principle of foreign policy in Europe and the United States. Whereas before Washington and its allies in Europe had to agree in order to keep the West united, now they are free to disagree and leave the West divided. Why this divergence? Mr. Kagan points to two mutually reinforcing factors.

One is that America is powerful and Europe is not: ''Strong powers naturally view the world differently than weaker powers. They measure risks and threats differently, they define security differently, and they have different levels of tolerance for insecurity.''

Iraq is a threat to Washington not only because Americans have a lower risk tolerance than Europeans, long accustomed to their own vulnerability, but also because America's overwhelming power gives it the means to do something about it.

Europe, on the other hand, lacks the power to eradicate the threat of Iraqi weapons of mass destruction, and it is therefore more inclined to rely on engagement and diplomacy rather than force.

''A man armed only with a knife may decide that a bear prowling the forest is a tolerable danger, inasmuch as the alternative -- hunting the bear armed only with a knife -- is actually riskier than lying low and hoping the bear never attacks,'' he writes. But give the man a rifle, and the calculation of risk changes.

The differences produced by the disparity of power are compounded by the very different historical experiences of the United States and Europe this past half century. As the leader of the ''free world,'' Washington provided security for many during a cold war ultimately won without firing a shot. The threat of military force and its occasional use were crucial tools in securing this success.

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Europe's experience has been very different. After 1945 Europe rejected balance-of-power politics and instead embraced reconciliation, multilateral cooperation and integration as the principal means to safeguard peace that followed the world's most devastating conflict. Over time Europe came to see this experience as a model of international behavior for others to follow.

''The transmission of the European miracle to the rest of the world has become Europe's new mission civilisatrice,'' Mr. Kagan writes. Rather than the threat of force and unilateralism, Europe believes conflicts are best resolved through peaceful diplomacy and multilateral engagement. Not war, but inspections is what will secure Iraq's disarmament. ''Thus we arrive at what may be the most important reason for the divergence in views between Europe and the United States,'' he writes. ''America's power and its willingness to exercise that power -- unilaterally if necessary -- constitute a threat to Europe's new sense of mission.''

Mr. Kagan marshals his arguments with care and precision. Contrary to the claims of pundits and politicians, the current crisis in United States-European relations is not caused by President Bush's gratuitous unilateralism, German Chancellor Gerhard Schröder's pacifism, or French President Jacques Chirac's anti-Americanism, though they no doubt play a part. Rather, the crisis is deep, structural and enduring.

Mr. Kagan's thesis has been justly celebrated on both sides of the Atlantic. But it has also come in for some notable criticism. That thesis is, to some extent, a caricature, as he readily admits. There are many Americans (not all of them Democrats) with views indistinguishable from Mr. Kagan's Europeans. And Britain's Tony Blair has made a better case for defeating terrorism and Iraq than has any American.

More problematic is Mr. Kagan's unidimensional conception of power. Though many Europeans may fail to appreciate that military power provides the foundation of security, many Americans fail to appreciate that lasting security requires more than brute force.

It also requires a commitment to uphold common rules and norms, to work out differences short of the use of force, to promote common interests through enduring structures of cooperation, and to enhance the well-being of all people by promoting democracy and human rights and ensuring greater access to open markets.

Mr. Kagan's focus on the military foundation of power tends to ignore these other dimensions of security, all of which are becoming more important at a time of increased globalization. Globalization is a force for both good and ill. It promotes the free flow of goods and ideas across borders, and thus enhances prosperity and undermines repressive regimes. But porous borders can also be exploited by terrorists, organized criminals, narcotics traffickers and money launderers bent on doing harm.

The consequences of globalization are beyond the powers of any one nation to control, even one as powerful as the United States. It requires far-reaching cooperation among many countries to promote globalization's many benefits and defeat its many ills. In the future, much as in the past, the United States and Europe will have to stand at the core of such cooperative efforts. Their challenge, therefore, is to overcome the sources of the differences Mr. Kagan so eloquently describes and forge a new partnership that can deal with the myriad threats and opportunities that define the new era.

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A version of this review appears in print on March 5, 2003, on Page E00001 of the National edition with the headline: BOOKS OF THE TIMES; Americans Are From Mars, Europeans From Venus. Today's Paper|Subscribe